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Re: Pulsed Tube Coils
On Mon, 6 Jan 1997 22:43:07 -0700 Tesla List <tesla-at-poodle.pupman-dot-com>
writes:
>Subscriber: FutureT-at-aol-dot-com Mon Jan 6 22:35:09 1997
>Date: Tue, 7 Jan 1997 00:03:16 -0500
>From: FutureT-at-aol-dot-com
>To: tesla-at-pupman-dot-com
>Subject: Re: Pulsed Tube Coils
>
>In a message dated 97-01-06 00:55:41 EST, you write:
>
><<
> >>snip
> >> >> I was thinking of building a tube coil using a pair of
> > >572-B's (basically heavy duty 811's) with a filtered DC
> > >> >plate supply of about 2500 volts, the power supply would
> > >have a output filter capacitance of about 15 UFD and the
> > > >>circuit would be the typical link coupled feedback setup.
> > >> But I was going to use a square or triangle wave signal
> > > >to drive a power mosfet (IRF-511) switch in the filament
> > >center tap (cathode return) to gate the tubes conduction
> > > >> much the same as what a variable rate/dwell rotary gap
> > >would do in a disruptive coil, or even use a audio
> > > >source to modulate a fixed bias reference in the cathode
> > >>return. Any thoughts on this approach?
> >
> > > >> Mark Graalman
> > >>
>>>>snip
>
> >> pulsed tube coil using this system will still require a tetrode
>and
> >>AC (or
> >> >other up-ramping voltage) on the screen grid for long spark
> >>production. I
> >> definitely recommend pursuing this cathode-type control technique.
>
> >If you
> >> >try this technique Mark, I'd definitely be interested in your
> >>results.
> >
> >> John Freau
> >
> >>Mark, All, (update)
> >
>> >snip
> >got a nice 18" staccato spark most of the time, but since I was
>still
>> >using
> >the non-sync, pulsed AC system, sparks sometimes "broke into
>multiple
> >>spikes"
> >and lost spark length, due to incorrect pulse timing.
> >
> > Next, I hooked up the DC power supply, and ran the system. I only
> >>turned up
> >the power part way so far (I don't like seeing my components burn
> >up!), but
> >>the sparks seemed to be maybe a little better than using by grid
> >controlled
> >DC staccato (neither case here using the "AC on the screen-grid"
> >voltage
> >>technique). What I mean by "better" is the sparks seemed less
> >"plasma-like".
>> > But it's hard to judge at low power.
>
> >>>>snip
>
>> >John Freau
> >
> >
> > Hi John,
>
>> How do you think it may work out if the screen is
>> supplied by a 500 -600 volt DC source rather than AC? Or
>> tying the cathode to ground through a 2 or 3 volt zener
> for a slight amount of bias and switching the screen volt
>> age on and off with the transistor switch? Of course I'm
> still talking about using DC on the plate circuit, I guess my ordinal
>> thoughts on this were to have a "silent"
> output discharge then work on different modulation
>> techniques. It will be some time before I can do much in
> the way of any experiments, my shop is loaded up, torn up, with
>> construction stuff from working on the house etc.
>> But the ideas still flow.......................
>
>> Mark Graalman (WB8JKR) TCBA#1399
>
>> BTW, what type of waveform are you using to control the
>> switching transistor?
>
> >>
> Mark,
>
>I knew I forgot something in my my posts---I used a squarewave for all
>tests--it's all my generator can provide. And Yes, I suppose that if
>the
>proper waveform is fed to the "cathode lifting" control transistor,
>this may
>make it unnecessary to use the AC on the screen grid. Then DC on
>screen grid
>would be OK, or use a triode. But if duty cycle is high, a lot of
>power will
>be drawn.
>
>BTW, I did try connecting my negative control pulse to the screen,
>instead of
>the control grid in one of my earlier experiments--and the technique
>worked
>well, and in fact eliminated the potential problem of interaction
>between the
>neg. control voltage and the grid-leak network.
>
>I also like your idea of using cathode bias to eliminate the need for
>a
>negative control voltage or screen voltage to cut off the tube. I'm
>not sure
>what zener voltage rating would be needed here, may need to be more
>than a
>few volts--if this is what you generally had in mind--and I suppose
>you'll
>lose power due to cathode degeneration. There are certainly plenty
>of
> possiblilites here.
>
>In general, when pulsed, a DC coil and an AC coil will sound about the
>same,
>this is because if only one half cycle is passed, the ear will hear
>this as a
>"pop" rather than a tone. depending on the pulse rate, the sound can
>be like
>a chain saw revving up, or like a rat-tat-tat, or staccato sound. If
>you
>use voice or music modulation, that's another story, and I can see why
>you
>would want the DC for a quiet coil starting point. I think the DC
>with audio
>modulation should work very well, but sparks will probably be short,
>and the
>coil will probably draw a lot of power, you should hear the audio
>coming out
>of the spark. I never tried this but it should be pretty cool.
>Perhaps
>someone else has tried this? The coil could be voice actuated, so
>the tube
>can cool between words, music will probably draw a lot of power, you
>may have
>to keep the power low, or use a giant tube to keep it cool, etc.
>
>It sounds like your onto something here with these ideas, I'll be
>interested
>to hear what you come up with once you get your lab back on line. I
>don't
>know just how much I'll be able to do along these lines, always glad
>to hear
>of any new ideas that you'd like to share.
>
>Happy (modulated) coiling,
>
>John Freau
>
Ok John,
Thanks for your input. I hope to get back into the swing of things this
summer.
As far as the cathode bias, with a High Mu tube like a
811A or 572 or 3-400/500 etc I would think that 5 to 8 volts would put it
at just about cutoff with a 2-3000 volt plate supply, a 50 watt zener and
a 100 ohm 20 watt
resistor would work well. As far as the bias voltage being degenerative,
bypass the bias diode/resistor with a
.1 ufd 1KV cap, this will be the RF path around the bias
network and it will cut way down on the RF degeneration.
Mark Graalman
TTYL!